This invention relates to techniques for assessing blood flow and, more particularly, to a method and apparatus for assessing perfusion and blood velocity in small vessels, such as those in the anterior segment of the eye.
The ability to image and quantify blood-flow is crucial in a wide range of medical specialties. Most of the ultrasound units used in radiology departments include the capability of performing color-flow Doppler imaging for this purpose. Depictions of color-flow produced by such conventional Doppler systems must inherently have poorer resolution than the gray-scale ultrasound image of stationary tissues over which the color-flow information is superimposed. The relatively poor resolution of Doppler color-flow limits the size and rate of flow detectable by such systems.
For example, understanding of the mechanisms of ophthalmic diseases including glaucoma and age-related macular degeneration as well as the mechanisms and effectiveness of treatment options has been limited by the lack of appropriate tools and techniques. The clinical significance of an improvement in the management of ophthalmic diseases alone is enormous since about three million people in the United States alone suffer from glaucoma, about one-hundred-thousand per year suffer some form of ocular trauma, and more than six million people in the United States suffer from degenerative retinal diseases. For example, while therapies for glaucoma have been developed, two years after the start of treatment, fifty-six percent of eyes treated first with laser and seventy percent of eyes treated first with medication needed new or extra medications to control pressure within the eye. The lack of appropriate tools and techniques has made it difficult to assess the efficacy of these treatment techniques.
High frequency ultrasound has shown promise for use as a clinical tool and technique, however practical limitations have restricted its use. For example, flow can be detected using a non-Doppler (time-domain) method known as an M-scan. An M-scan consists of a series of vectors acquired at fixed time intervals along one line of sight. In an M-scan, fixed tissue interfaces remain the same distance from the (stationary) transducer (a transducer is a device that emits an acoustic pulse in response to a voltage transient and converts echoes into electrical signals). But if a scatterer (a scatterer is capable of reflecting or scattering ultrasonic energy having an acoustic impedance (density x speed-of-sound) different from the surrounding medium), such as a blood cell conglomeration, is in motion along the line of sight, its range will change with time. One example of a time-domain technique for mapping flow based on acquisition of a series of spatially offset M-scans is disclosed in Ferrara, K. W., et al., xe2x80x9cEstimation of Blood Velocity With High Frequency Ultrasound,xe2x80x9d IEEE Trans Ultra Freq Cons., 43:149-157 (1996) which is herein incorporated by reference. By combining a series of M-scan determinations at independent adjacent spatial positions which are spaced at distances greater than the lateral resolution of the ultrasound beam as shown in FIG. 1A, B-mode images with flow information can be produced. At each spatial position, groups of moving blood cells are detected and their range determined in successive vectors, from which their velocity is computed. When the data are acquired, two-dimensional (xe2x80x9c2-Dxe2x80x9d) matrix and three-dimensional (3-D) flow maps can be produced using techniques, such as the one disclosed in Stith, A., et al., xe2x80x9c3-D Ultrasonic Mapping of the Microvasculature,xe2x80x9d Proc IEEE Ultrason Symp., 1473-1476 (1996) which is herein incorporated by reference.
A significant factor limiting the clinical utility of this technique arises from the intermittent nature of the scanning procedure. To scan a diagnostically useful lateral range, M-scan sequences must be acquired at approximately one-hundred spatially independent positions. For each of these positions, transducer motion must be initiated, motion stop confirmed, and data acquired and stored. These operations are time consuming, easily approaching 0.5 seconds per position and expending as much as one minute for a single plane. In the case of the eye, particularly, voluntary and involuntary motions over such a long period are inevitable.
Previous attempts to measure blood flow within the eye using conventional color Doppler ultrasound methods have also been limited by insensitivity to very slow velocities ( less than 1.5 cm/s) as disclosed in T. H. Williamson and A. Harris, xe2x80x9cColor Doppler ultrasound imaging of the eye and orbit,xe2x80x9d Survey of Ophthalmology, vol. 40, pp. 255-267, 1996 which is herein incorporated by reference and the inability to resolve vessels smaller than 300 microns. Studies have demonstrated the ability to assess blood flow in the ophthalmic artery and vein and in the short posterior ciliary artery, however these vessels are generally larger and contain higher flow velocities compared to those found in the anterior segment as disclosed in A. Harris, L. Kagemann, and G. A. Cioffi, xe2x80x9cAssessment of human ocular hemodynamics.xe2x80x9d Survey of Ophthalmology, vol. 42, pp. 509-533, 1998 and T. H. Williamson and A. Harris, xe2x80x9cOcular blood flow measurement.xe2x80x9d British Journal of Ophthalmology, vol. 78, pp. 939-945, 1994 which are herein incorporated by reference. While studies using high frequency ultrasound demonstrate the ability to resolve structures down to forty microns in the anterior segment of the eye as disclosed in C. J. Pavlin, D. A. Christopher, P. N. Burns, and F. S. Foster, xe2x80x9cHigh-frequency Doppler ultrasound examination of blood-flow in the anterior segment of the eye.xe2x80x9d American Journal of Ophthalmology, vol. 126, pp. 597-600, 1998, and such B-scans of the eye are clinically useful in diagnosing diseases, such as melanoma of the ciliary body and open angle glaucoma as disclosed in C. J. Pavlin, xe2x80x9cPractical application of ultrasound biomicroscopy.xe2x80x9d Canadian Journal of Ophthalmology, vol. 30, pp. 225-229, 1995, K. J. Coleman, S. Woods, M. J. Rondeau, and R. H. Silverman, xe2x80x9cOphthalmic ultrasonography.xe2x80x9d Radiologic Clinics Of North America, vol. 30, pp. 1105-1114, 1992, and C. J. Pavlin, K. Harsiewicz, M. D. Sherar, and F. S. Foster, xe2x80x9cClinical use of ultrasound biomicroscopy.xe2x80x9d Ophthalmology, vol. 98, pp. 287-295, 1991 which are herein incorporated by reference, these studies have difficulties with clutter discrimination, resolution, and possibly energy levels.
A method of assessing blood flow in a tissue in accordance with one embodiment of the present invention includes sequentially directing a beam through the tissue along overlapping lines of sight and then generating blood flow data from echo data from where the beams overlap to evaluate the blood flow in the tissue.
A method of measuring blood flow velocity in at least one vessel in a tissue in accordance with another embodiment of the present invention includes a few steps. First, a beam is sequentially directed through the vessel in the tissue along overlapping lines of sight. Next, blood flow data are generated from echo data from where the beams overlap. Finally, the blood flow velocity in the vessel is determined based on the generated blood flow data.
A method of analyzing a vessel in a tissue in accordance with another embodiment of the present invention includes a few steps. First, a beam is sequentially directed through the vessel in the tissue along overlapping lines of sight. Next, blood flow data are generated from echo data from where the beams overlap. Finally, an image of the vessel is provided based on the generated blood flow data.
An apparatus for assessing blood flow in accordance with one embodiment of the present invention includes a transmission system and a storage system. The transmission system generates a beam which is sequentially transmitted towards the tissue along a plurality of overlapping lines of sight and receives echo data from the transmission along each of the lines of sight. The storage system stores the echo data from the transmission system for each of the lines of sight.
In contrast to prior systems in which an ultrasonic pulse was repeatedly directed to a discrete line-of-sight, the present invention continuously scans over a region in order to rapidly assess blood velocities in blood vessels. Using this invention, a transducer can rapidly translate a beam across a region of interest in an overlapping pattern and sensitive maps of blood velocity in blood vessels can be constructed. As a result, this invention provides an effective and quick, typically less than two seconds, method and apparatus for visualizing and measuring blood flow in a variety of different regions of the body, including vessels smaller than 300 microns and at blood velocities less than 1.5 cm/sec.
One particularly important application of this new mode is in the evaluation of the functionality of the iris, ciliary body and ciliary processes which are located in the anterior segment of the eye. These structures are highly vascular, and share a common blood supply. The iris is a muscular structure, pigmented anteriorly, which controls the aperture size of the pupil, and thus the amount of light focused onto the retina. The ciliary body is a muscular structure in the anterior portion of the eye, responsible for accommodation, and the ciliary processes, which extend off of the ciliary body, produce aqueous fluid. Because these are opaque tissues, they have been inaccessible to prior optical methods, and with vessels smaller than 300 microns, they have been inaccessible to prior color flow mapping systems. With the invention, a 2-D scan of the eye can be obtained in an interval on the order of about one second, and blood flow through the iris and ciliary body can be detected in vessels down to at least forty microns. Assessing blood flow in small vessels that supply structures in the anterior segment, such as in the long posterior ciliary artery or the major arterial circle, and evaluating the response to disease mechanisms and therapeutic interventions is now possible with this invention and is an important step in determining the health of the eye.
Yet another advantage of this invention over prior techniques is that a filter can be applied continuously to the return of the ultrasonic beam or pulse from all regions. As a result, the transient response that occurs along each line-of-sight in traditional Doppler systems can be eliminated.